The present invention relates to a process of and a system for liquefying air for separating components thereof and, more particularly, to a process and system for the above stated purpose employing a compound fractionating column for allowing the separation at a relatively low pressure.
Nowadays, systems are used in various fields of industry, for liquefying air to separate nitrogen, oxygen, argon and other components. These systems employ, in order to facilitate the liquefaction, compressors for compressing the feed air. These compressors consume very large driving power and, accordingly, cost a great deal. It is not too much to say that the operation cost for the air compressor is the cost for operating the whole system.
Especially, in the systems for fractionating air, which are usually of large scale and capacity, the problem of this operation cost is correspondingly serious, and for this reason, they are keen to reduce the operation cost of the system.
It has been recognized that the power cost for an air compressor is mterially in proportion to the delivery pressure of the compressor, i.e. the pressure of the feed air. Therefore, the power cost for the air-liquefaction plant can be reduced by lowering the pressure of the feed air.
In order to separate oxygen of high purity from the compressed and cooled material air, it is necessary to use a so-called compound fractionating column having a medium pressure column for performing a primary fractionation of the feed air and a low pressure column for effecting a subsequent secondary fractionation.
In general, for obtaining a satisfactory operation of the compound fractionating column, it is indispensable that upward flow of gas and returning liquid are maintained in the medium pressure and low pressure columns, since the fractionation is caused by gas-liquid contact of the upwardly flowing gas and the returning liquid on a plurality of stages of plate.
Thus, the conventional techniques can be sorted into two types by the ways for obtaining the upward flow of gas and the returning liquid flow in the low pressure and medium pressure columns.
To explain the technique of the first type, the feed air in gaseous phase (this may be partially liquefied in the preceding process) is introduced into a medium pressure column through the bottom of the latter to form an upward flow of gas therethrough. The gas makes contact with returning liquid on the stages of plates as it flows upwardly to be fractionated, and becomes nitrogen gas of a considerable purity when it reaches the top of the medium pressure column. Meanwhile, liquefied oxygen of a high purity is obtained at the bottom of the low pressure column. The liquefied oxygen available at the bottom of the low pressure column and the nitrogen gas obtained at the top of the medium pressure column are brought into a condenser for a heat exchange therebetween, to condense the nitrogen gas and to evaporate the oxygen. The condensate of the nitrogen is used as the returning liquid in the medium pressure column. At the same time, a part of the condensate is introduced, through a pipe having an expansion valve, to the top of the low pressure column to be used as the returning liquid through the low pressure column.
The evaporated oxygen at the bottom of the low pressure column then flows upwardly through the low pressure column as the upward flow of gas. The oxygen makes contact, in the course of the flow to the top of the column, with the returning liquid on the stages of plate. Consequently, a gas rich in nitrogen, i.e. a nitrogen gas of not so high purity, is obtained at the top of the low pressure column. Thus, the gas rich in nitrogen is picked out from the top of the column, while the returning liquid reached the bottom of the column in the form of liquified oxygen of high purity is taken out as the industrial product.
Thus, the technique of method of the first type depends on a heat exchange between the liquefied oxygen obtained at the bottom of the low pressure column and the nitrogen gas obtained at the top of the medium pressure column. In order that this heat exchange is performed satisfactorily, it is necessary to maintain a pressure in the medium pressure column high than that in the low pressure column, so as to make the saturating temperature of the nitrogen gas at the top of the medium pressure column higher than the saturating temperature of the liquefied oxygen at the bottom of the low pressure column.
In general, the pressure in the low pressure column is about 1.3 Kg/cm.sup.2, while the temperature difference between two phases in the condenser is about 2.degree. K. Therefore, the temperatures of the liquefied oxygen and the nitrogen gas are typically 92.degree. K. and 94.degree. K., respectively. Consequently, the medium pressure column must be maintained at a high pressure of about 5.2 Kg/cm.sup.2. Thus, the pressure of the feed air is substantially same to that of the medium pressure tower.
This means that the method of the first type in which the pressure of the feed air is high is unacceptable due to the high power cost.
In order to avoid the above described inconvenience, the method of second type has been proposed as follows.
The feed air is devided into two volumes, one of which is introduced into a condenser provided at the bottom of the low pressure column, for a heat exchange with the liquefied oxygen available thereat. The condensate air is then introduced into a middle section of the medium pressure column. The oxygen evaporated in the condenser is then used as the upward flow of gas through the low pressure column. Meanwhile, the remainder part of the feed air is introduced to the bottom of the medium pressure column to be used as the upward flow of gas therethrough.
The returning liquids for the medium pressure and low pressure columns are obtained in accordance with the following measure. Namely, at first a liquefied gas having a boiling point lower than that of the liquefied oxygen at the bottom of the low pressure column is prepared at the outside of the apparatus. The gas is introduced to a condenser provided at the top of the medium pressure column, for a heat exchange with the nitrogen gas at the top of the medium pressure tower. Consequently, the nitrogen is condensated to liquid phase, a part of which is used as the returning liquid through the medium pressure column while the remainder is introduced to the top of the low pressure column to flow therethrough as the returning liquid. The liquefied gas is evaporated as a result of the heat exchange at the condenser and is extracted out of the system.
According to this method of the second type, the pressure of the nitrogen gas at the top of the medium pressure column, i.e. the pressure of the feed air can be determined by the boiling point of the liquefied gas separately prepared externally of the system. Supposing here that liquefied oxygen opened to atmospheric is used as the liquefied gas for the heat exchange, the temperature of the nitrogen gas at the top of the medium pressure column may be 92.degree. K., since the boiling point of liquefied oxygen under atmospheric pressure is 90.degree. K., so that pressure at the top of the medium pressure tower, i.e. the pressure of the feed air can be as low as about 4 Kg/cm.sup.2.
Althogh the method of the second type can make use of the material air of a pressure reduced by about 1 Kg/cm.sup.2 as compared with that of the first type method, it suffers from the following problems because of the separate preparation of the heat exchanging liquefied gas externally of the system.
The first problem resides in that the system necessitates the heat exchanging gas which is to be liquefied by another independent system. It is of course possible to make use of the oxygen liquefied at the bottom of the low pressure column as the heat exchanging liquefied gas. However, such a measure would cause a shortage of the cold heat source, resulting in a deterioration of the smooth operation of the system, when an excessively large amount of liquefied oxygen is extracted for the purpose of above stated heat exchange.
There exists a practical lower limit for the amount of the liquefied oxygen condensate at the top of the medium pressure tower, for an efficient separation and recovery of the pure oxygen from the feed air.
For instance, even when the feed air is perfectly fractionated, i.e. by 100%, the amount of oxygen collected at the bottom of the low pressure column is 21% with respect to the amount of the feed air. The amount of the cold heat source for condensing the nitrogen at the top of the medium pressure column by an amount sufficiently large to provide enough returning liquids for both columns usually exceeds the cold heat discharged from the liquid oxygen the amount of which is 21% of the feed air at the largest.
Therefore, for obtaining a smooth operation of the compound fractionating column, it is necessary to prepare a liquefied gas produced in a separate system, is addition to the liquefied oxygen produced in the system in question itself, for use as a cold source.
In addition, when the oxygen product must be taken out in the liquid phase, the amount of liquefied oxygen available for the heat exchange gets smaller accordingly. Thus, a larger amount of liquefied gas must be prepared in the separate system.
The second problem resides in that the continuous operation of the liquefaction system for the air is rendered unstable. As is well known, a considerably long time is required for starting the system. Once the system is started, it is operated continuously for a half year or longer, for maintaining the loss of cold energy and other reasons. Therefore, the liquefied gas for the heat exchange must be always prepared over the long period of operation of the system. It involves various problems to rely this continuous preparation of the liquefied gas upon the separate system. Consequently, the stable operation of the system is often failed.
The third problem resides in that the air fractionating system becomes unacceptably complicated. Since a separate system is required for preparing the liquefied gas for the purpose of the heat exchange, transportation and storage equipments must be employed additionally, to make the fractionating system as a whole complicated. In addition, the additional provision of the transportation and storage equipments inevitably increases the loss of cold energy.
Thus, in the conventional refractionating method of the second type, the advantage over the method of the first type brought about by the reduced power cost is negative by the disadvantage attributable to the necessity of the additional liquefied gas and the separate system for preparing the additional liquefied gas.
Under these circumstances, the present invention aims at providing an improved process of and system for liquefying air and separating its components, at a reduced power cost and without necessitating the additional separate system.
According to the first aspect of the invention, there is provided a process in which feed air which has been compressed by an air compressor and cooled by a heat exchanger is divided into three portions and a first portion of the feed air is supplied through an expansion turbine to a low pressure column, while a second portion of the feed air is introduced to a medium pressure column and is utilized as the upward flowing gas in the medium pressure column which is brought into contact with returning liquid obtained in a first condenser, so that nitrogen gas is obtained at the top of the medium pressure column. Meanwhile, the returning liquid is changed into liquefied air as it reaches the bottom of the medium pressure column. At the same time, a third portion of the feed air is introduced into a second condenser for a heat exchange with liquefied oxygen available at the bottom of the low pressure column, so as to evaporate the oxygen. The resulted oxygen gas then flows through the low pressure column upwardly contacting the returning liquid falling down from the top of the column, and is taken out from the system, as an impure gas, from the top of the low pressure column, while the returning liquid is changed into liquefied oxygen as it travels downwardly to the bottom of the low pressure column. The process of the invention is characterized, further to the above stated features, by a fact that the liquefied air at the bottom of the medium pressure column is expanded to have its pressure reduced to a low pressure level substantially equal to the pressure level in the low pressure column and introduced into the first condenser, and the expanded liquefied air of low pressure is subjected to heat exchange with the nitrogen gas at the top of the medium pressure column, so that the liquefied air will be vaporized and the nitrogen gas will be condensed for making use of the latter as the returning liquid for the medium pressure column.
According to another aspect of the invention, there is provided a system for liquefying air to separate its components including a medium pressure column adapted to invite a second portion of the feed air divided into three portions to allow it to flow upwardly therethrough, for putting it into contact with downfalling returning liquid, so as to fractionate the feed air in the form of upward flow of gas, thereby to obtain nitrogen gas and liquefied air, at the top and bottom thereof, respectively, a first condenser adapted to perform a heat exchange between a liquefied air of reduced pressure obtained by expanding the liquefied air at the bottom of the medium pressure column to have its pressure reduced to a low pressure level substantially equal to the pressure level in a low pressure column and the nitrogen gas available at the top of said medium pressure column, thereby to condense the latter for use as the returning liquid for said medium pressure column, a second condenser for receiving a third portion of the feed air for effecting heat exchange between it and liquefied oxygen, so as to evaporate the oxygen, and a low pressure column in which an upward flow of gas constituted by the oxygen evaporated in said second condenser and the gaseous air obtained by vaporization in the first condenser and introduced into the low pressure column is fractionated through a content with a downfall returning liquid from the top of said low pressure column, and the portion of the liquefied air introduced into the low pressure column after being expanded to have its pressure reduced to a low pressure level substantially equal to the pressure level in the low pressure column so as to obtain an impure gas to be taken out from the top of said lower pressure column and change said returning liquid into liquefied oxygen as it travels downwardly to the bottom of said low pressure column.
These and other objects, as well as advantageous features of the invention will become clear from the following description of the preferred embodiment taken in conjunction with the attached drawings in which: